It is well known that the encapsulation of active substances offers important advantages such as, for example, the protection of the substance or its slow or delayed release at the site of use.
For applications in the cosmetic, pharmaceutical or food sectors, the materials which are most widely sought after as constituents of the wall are natural substances, particularly proteins or polysaccharides, because of their biocompatibility.
Some microencapsulation processes using proteins or polysaccharides derive from the interfacial polycondensation method described by Chang (Chang T. M. S., Science, 1964, 146, 524-525). According to this well-known method, an aqueous solution of a diamine is emulsified in a hydrophobic phase, and a solution of a diacid chloride is then added to the emulsion. The amine groups form amide linkages with the acid dichloride at the interface to give a membrane which individualizes microcapsules. It is known that the polycondensation reaction is promoted by the alkalization of the aqueous phase since the reaction releases hydrochloric acid, which, in the absence of an alkaline agent, protonates some of the amine groups and thereby makes them non-acylatable. The same applies when the diamine is replaced with a protein in order to form microcapsules of crosslinked protein. In the processes described in the literature of the prior art, the initial aqueous solution of protein is alkalized so that all the free amine groups of the protein are in an acylatable, non-protonated form. Thus, for example, the document U.S. Pat. No. 4,780,321 in the name of LEVY relates to the preparation of microcapsules with mixed walls formed of crosslinked polysaccharides and proteins. The initial aqueous solution of polysaccharide and protein is alkaline, the pH preferably being &gt;10. Likewise, in the document U.S. Pat. No. 5,395,620 in the name of HUC, which relates to microcapsules with a wall of crosslinked atelocollagen and glycosaminoglycans, the pH of the initial aqueous solution is preferably basic, all the Examples in said patent using a carbonate buffer of pH 9.8.
In the document FR-A-2,444,497 in the name of Mars, an aqueous solution of protein is used without the addition of alkaline substances or buffers. However, the aqueous phase contains a high concentration of protein, equal to at least 20% (w/w), so that part of it serves to neutralize the HCl formed (buffer role) while the non-protonated fraction can be used to form the membrane.
Glutaraldehyde can also be used to manufacture particles from proteins. The crosslinking is generally effected in a neutral or alkaline medium. Thus, for example, serum albumin can be crosslinked in 20% solution in a phosphate buffer of pH 7.5 (Sheu M. T. et al., J. Parenter, Sci. Technol., 1986, 40, 253-258) with 1% of glutaraldehyde. However, the mechanism of the reaction of glutaraldehyde with the proteins is complex and remains poorly elucidated. It is known that the reaction involves not only the free glutaraldehyde but also polymeric forms originating from the condensation of glutaraldehyde with itself, it being possible for these derivatives to exist in linear or cyclic form. The composition of the glutaraldehyde solutions in terms of these different reactive forms is variable and depends on diverse factors, so the nature of the bonds formed and the degree of crosslinking are not completely controlled; this is an obstacle to industrial development (Saleh A. M. et al., Int. J. Pharm., 1989, 57, 205-210). Furthermore, this very reactive crosslinking agent can interfere with various active principles and thereby reduce their bioavailability (Gupta P. K. and Hung C. T., J. Microencapsulation, 1989, 6, 427-462). Finally, free aldehyde groups can be present on the particles (Magee G. A. et al., J. Controlled Release, 1995, 37, 11-19). The presence of such reactive groups on particles intended for human use is not desirable. For example, toxic effects of empty particles have been observed on macrophages (Suunders J. et al., Int. J. Pharm., 1991, 68, 265-270).
The proteins mentioned in the documents of the literature of the prior art are animal proteins, none of them mentioning the use of plant proteins.
If the conditions described in the documents of the prior art, where the aqueous phase used to dissolve the plant proteins is a sodium carbonate buffer of pH 9.8, are applied to commercially available plant protein preparations, it is not possible to obtain stable microcapsules. The microcapsules are obtained in very small quantities. They have a fragile membrane: some of them often appear to be open under microscopic examination. They form numerous aggregates and deteriorate very rapidly in a few days at 45.degree. C. in the form of an aqueous suspension. The same applies when phosphate buffers with a pH of between 7 and 8, or simply distilled water, are used to dissolve the plant proteins.
Similarly, stable microcapsules are not obtained if liquid preparations containing plant proteins, such as commercial soya milks, are used directly without being buffered, or if they are buffered by the addition of sodium carbonate or sodium phosphate.